1. Field of the invention:
This invention relates to an adhesive composition for use in the mounting of electronic parts on a printed circuit board, and it also relates to a method for mounting electronic parts on a printed circuit board by the use of the adhesive composition.
2. Description of the prior art
In recent years, when electronic parts such as resistor chips and condenser chips are to be soldered on a printed circuit board, these electronic parts are first fixed on the printed circuit board by the use of a photosetting adhesive composition (see, for example, Japanese Laid-Open Patent Publication No. 58-180090). In such a method, the photosetting adhesive composition is applied to the positions intended for the electronic parts on the printed circuit board, and the photosetting adhesive composition is then made more viscous by the application of light, after which the electronic parts are pressed onto the applied adhesive composition, resulting in a fixing of the electronic parts. When the photosetting adhesive composition is applied, the viscosity of the adhesive composition remains low, so that the operation of application can be readily conducted with the use of a device for application such as a dispenser or the like. When the electronic parts are to be fixed, the flowability of the adhesive composition is made to decrease and the adhesion of the composition is made to increase by the increase in the viscosity of the photosetting adhesive composition, so that the electronic parts will not move out of position.
However, this method has disadvantages in that it is difficult to control the reaction at the time of increasing the viscosity of the adhesive composition by the application of light. That is, when the reaction proceeds too far, the adhesion of the composition decreases or disappears. On the other hand, when the reaction does not proceed far enough, the viscosity of the composition does not increase, so that the electronic parts may readily move out of position.
In order to eliminate these disadvantages, the adhesive composition may be not only photosetting but also thermosetting. The degree of photosetting of this adhesive composition is such that the increase in the viscosity of the composition caused by the application of light is sufficient to fix electronic parts to the desired positions and the curing reaction of the composition will not proceed further. The adhesive composition which has been treated with light to increase its viscosity can be completely cured by the application of heat. This makes it possible to increase the heat resistance of the composition and to prevent the sticking of dust to the composition. In some cases, even if the degree of photosetting of the adhesive composition is controlled as described above, the curing reaction proceeds too far when light is being applied, so that the adhesion of the composition often decrease or disappears.
When a conventional adhesive composition for use in the mounting of electronic parts is used, as shown in FIG. 14, and when UV light is used to treat this adhesive composition SO applied to the surface of a printed circuit board 1, the UV light reaches into the inner portion of the adhesive composition 30, so that all of the adhesive composition 30 gels. The adhesive composition 30 hardens in the shape of a mound, and when each of the electronic parts 4 and 6 is pressed onto this galled adhesive composition 30, the shape of the gelled adhesive composition 30 does not change. Therefore, the electronic parts 4 and 6 remain above the printed circuit board 1 at some distance from the wiring pattern on the printed circuit board 1. Also, the area of contact between the adhesive composition 30 and the electronic parts 4 and 6 is small, so that the adhesion of the composition 30 is not enough to fix the electronic parts 4 and 6 on the printed circuit board 1.
The inventors of this invention found that when an ultraviolet lamp was used, not only ultraviolet light is emitted, but also heat was generated, and that this heat caused the curing of the adhesive composition. This problem arises because the adhesive composition is sensitive to light and heat. By the control of the amount of light emitted from a lamp, it should be possible to overcome this difficulty. However, the amount of light emitted from a lamp is always varying, and hence, it is not possible to control strictly the amount of light to which the adhesive composition is exposed.
The curing property of the adhesive composition is easily affected by light or heat, and the stability of the composition during storage is poor.
If the adhesive composition can be applied to a printed circuit board by a painting technique, the finished mounting condition of the electronic parts will be extremely satisfactory. However, the viscosity of the adhesive composition would have to be much smaller than that of currently available adhesive compositions. In order to decrease the viscosity of the adhesive composition, it may contain a solvent, which raises the problem of toxicity. If the amount of filler in the adhesive composition is decreased, the relative amount of resin ingredients used is increased, resulting in increased cost.